1) Field of the Invention
The present invention relates to a fluorescence lifetime measurement apparatus that calculates a fluorescence lifetime by measuring the number of fluorescence photons emitted from a specimen which is irradiated with excitation light.
2) Description of the Related Art
There is known a method of calculating a fluorescence lifetime by irradiating a specimen with excitation light to excite the specimen, and by measuring the number of fluorescence photons emitted from the specimen. In particular, in a field of biochemistry, attention is paid to a fluorescence lifetime measurement apparatus using pulse excitation light because a microstructure of a living body can be clarified by creating a fluorescence lifetime distribution image. As a method of calculating the fluorescence lifetime, there is known a time correlated single photon counting (TCSPC) in which a specimen is irradiated with pulse excitation light and a time until fluorescence photons are received is measured (see Japanese Patent Application Laid-Open No. 2002-107300).
There is also known a time gate method in which (1) a specimen is irradiated with pulse excitation light, (2) the number of fluorescence photons emitted from the specimen in a plurality of time windows each referred to as “a time gate” is measured, and then (3) the fluorescence lifetime is calculated from the number of fluorescence photons measured.
The TCSPC requires that the number of fluorescence photons emitted by the irradiation of one pulse excitation light be a very small number of about 0.01 piece. Accordingly, the specimen must be irradiated with pulse excitation light at least several tens of thousands of times to calculate the fluorescence lifetime once, and thus it takes a very long time to calculate of the fluorescence lifetime.
On the other hand, the time gate method can effectively use pulse excitation light. However, setting a plurality of time gates inappropriately makes a large error in a calculated fluorescence lifetime. Therefore, the time gates must be appropriately set to reduce the error of the fluorescence lifetime. Thus, when a fluorescence lifetime of an unknown specimen is measured in the time gates fixed, if the time gates fixed are inappropriate, it is difficult to reduce the error of the fluorescence lifetime.